Fartlek

In an article in Athletics Journal, Paul A. Smith described fartlek as "a continuous overdistance run with numerous faster-paced interval runs interspersed, until the runner feels tired, but not exhausted." Smith claimed that because fartlek existed in the mind of the runner as a form of play, it deemphasized the feeling or perception of fatigue. […] In a typical fartlek workout, you pick some landmark such as a tree or a bush and sprint to it, then jog until you've recovered.

Interval workouts are typically more intense and structured than strides and fartleks. If you have access to a track, distance is easier to gauge.